This invention refers to ultrasonic pulse-echo testing in which an ultrasonic search signal is transmitted cyclically from a transducer probe into a workpiece and the probe subsequently senses the receipt of echo signals arising from the search signal intercepting an acoustic discontinuity which reflects a portion of the transmitted acoustic energy.
As is well known to those skilled in the art great progress has been made in improving the resolution capability available using pulse-echo test apparatus with respect to small defects and defects disposed at relatively small distances from one another. These improvements are largely due to improved transducer construction and the replacement of electron tube amplifiers by transistorized amplifiers. Nevertheless, there is still a minimum distance in which a defect close to the workpiece surface cannot be resolved. This lack of resolution is due to the ringing of the transducer subsequent to it being pulsed for transmitting the search pulse, such ringing masking the echo signal. Also, if the transducer is excited by the receipt of a defect responsive acoustic signal, an immediately following similar defect responsive signal may be masked by the response of the transducer to the firstoccurring echo signal. Hence, the second defect responsive signal is not recognized. These cases clearly illustrate the lack of resolution still prevalent in currently used pulse-echo ultrasonic test apparatus.
In order to stop the ringing of the transducer probe after being excited by the transmit pulse, it has been proposed to immediately apply a pulse of opposite polarity so as to stop the oscillatory motion of the piezoelectric material. This method, by virtue of its complexity, has never been successful. Increased damping of the transducer by mechanical or electrical means obviously reduces its response to small signals and thereby decreases the transducer sensitivity. Similarly, pulse shortening circuits in the form of heretofore used filters decrease the sensitivity of the test apparatus and are of little usefulness in solving the problem of lack of resolution.